The purpose of this study was to investigate the possible factors that may influence the Doppler frequency spectrum when an ultrasound contrast agent is used to enhance the signal. Computer simulations of limited frequency resolution and attenuation over the sample volume has been compared to in vitro measurements. The results show that the limited frequency resolution may influence the maximum velocity estimate and the attenuation over the sample volume is of less clinical importance. An ultrasound contrast agent's main influence on a Doppler measurement is a pure signal enhancement